[Auth] Remove deprecated pre-iOS 13.5 GameCenter workaround and fix force-unwrap

FirebaseAuth/Sources/Swift/AuthProvider/GameCenterAuthProvider.swift

@@ -16,17 +16,6 @@
   import Foundation
   import GameKit
 
-  // TODO: Delete this when minimum macOS version passes 10.15.5.
-  /// WarningWorkaround is needed because playerID is deprecated in macOS 10.15.0 but still needed
-  /// until
-  /// 10.15.5 when the fetchItems API was introduced.
-  private protocol WarningWorkaround {
-    static func pre135Credential(localPlayer: GKLocalPlayer,
-                                 completion: @escaping (AuthCredential?, Error?) -> Void)
-  }
-
-  extension GameCenterAuthProvider: WarningWorkaround {}
-
   /// A concrete implementation of `AuthProvider` for Game Center Sign In. Not available on watchOS.
   @objc(FIRGameCenterAuthProvider) open class GameCenterAuthProvider: NSObject {
     /// A string constant identifying the Game Center identity provider.
@@ -51,60 +40,23 @@
         return
       }
 
-      if #available(macOS 10.15.5, *) {
-        localPlayer.fetchItems { publicKeyURL, signature, salt, timestamp, error in
-          if let error = error {
-            completion(nil, error)
-          } else {
-            let credential = GameCenterAuthCredential(withPlayerID: "",
-                                                      teamPlayerID: localPlayer.teamPlayerID,
-                                                      gamePlayerID: localPlayer.gamePlayerID,
-                                                      publicKeyURL: publicKeyURL,
-                                                      signature: signature,
-                                                      salt: salt,
-                                                      timestamp: timestamp,
-                                                      displayName: localPlayer.displayName)
-            completion(credential, nil)
-          }
+      localPlayer.fetchItems { publicKeyURL, signature, salt, timestamp, error in
+        if let error = error {
+          completion(nil, error)
+        } else {
+          let credential = GameCenterAuthCredential(withPlayerID: "",
+                                                    teamPlayerID: localPlayer.teamPlayerID,
+                                                    gamePlayerID: localPlayer.gamePlayerID,
+                                                    publicKeyURL: publicKeyURL,
+                                                    signature: signature,
+                                                    salt: salt,
+                                                    timestamp: timestamp,
+                                                    displayName: localPlayer.displayName)
+          completion(credential, nil)
         }
-      } else {
-        (GameCenterAuthProvider.self as WarningWorkaround.Type).pre135Credential(
-          localPlayer: localPlayer, completion: completion
-        )
       }
     }
 
-    @available(iOS, deprecated: 13.0)
-    @available(tvOS, deprecated: 13.0)
-    @available(macOS, deprecated: 10.15.0)
-    @available(macCatalyst, deprecated: 13.0)
-    fileprivate class func pre135Credential(localPlayer: GKLocalPlayer,
-                                            completion: @escaping (AuthCredential?, Error?)
-                                              -> Void) {
-      localPlayer
-        .generateIdentityVerificationSignature { publicKeyURL, signature, salt, timestamp, error in
-          if error != nil {
-            completion(nil, error)
-          } else {
-            /**
-             `localPlayer.alias` is actually the displayname needed, instead of
-             `localPlayer.displayname`. For more information, check
-             https://developer.apple.com/documentation/gamekit/gkplayer
-             **/
-            let displayName = localPlayer.alias
-            let credential = GameCenterAuthCredential(withPlayerID: localPlayer.playerID,
-                                                      teamPlayerID: nil,
-                                                      gamePlayerID: nil,
-                                                      publicKeyURL: publicKeyURL,
-                                                      signature: signature,
-                                                      salt: salt,
-                                                      timestamp: timestamp,
-                                                      displayName: displayName)
-            completion(credential, nil)
-          }
-        }
-    }
-
     /// Creates an `AuthCredential` for a Game Center sign in.
     @available(watchOS 8, *)
     open class func getCredential() async throws -> AuthCredential {
@@ -113,7 +65,7 @@
           if let credential = credential {
             continuation.resume(returning: credential)
           } else {
-            continuation.resume(throwing: error!) // TODO: Change to ?? and generate unknown error
+            continuation.resume(throwing: error ?? AuthErrorUtils.error(code: .internalError))
           }
         }
       }